The present invention relates to a method and apparatus for automatically measuring a dimension of an object onto which electron beam is radiated and which emits secondary electrons with high precision.
A conventional method and apparatus for measuring a small dimension or size is known. Such a small dimension includes a dimension of crystals of a metal, a dimension of an subgrain boundary, a dimension of a non-metallic inclusion in a metal, a dimension of an alloy phase, a powder particle size, a dimension of an IC mask pattern, a size of a CRT spot, a dimension of an insulating ceramic, a dimension of a color TV shadow mask, a pore size and a void size of an electrode coke, a gap dimension of a magnetic core, a gap dimension of a magnetic head and the like. An example of a pattern formed on a semiconductor wafer will be described hereinafter.
According to a conventional method for measuring a width of a photoresist pattern, a polysilicon pattern, a silicon nitride pattern and any other various types of pattern formed on a semiconductor wafer, an image of a pattern as an object to be measured is optically enlarged and measured using a micrometer with an optical microscope, electronic measuring equipment combining an industrial television and an optical microscope, or the like. According to another conventional method, a laser beam is irradiated onto a semiconductor wafer using a device comprising a combination of a laser and a precision-movable stage, and a width of a pattern is measured by measuring the intensity of reflected light and the amount of displacement of the stage.
According to still another conventional method, using a measuring device with a scanning electron microscope (to be referred to as SEM hereinafter), an enlarged image of a pattern of a semiconductor wafer is measured using a scale, and the width of the pattern is determined from the obtained measurement and the magnification of the SEM during the measurement. Still another conventional method is also known wherein an enlarged image of a pattern is divided into a plurality of picture elements, cursors are generated on a screen, the operator aligns the cursors at the edges of the pattern on the screen, and the dimension of the pattern to be measured is determined from the number of picture elements between the cursors and the magnification of the SEM. Still another conventional method is also described in Japanese Patent Disclosure No. 56-105633. According to this method, a signal obtained from the SEM and corresponding to the semiconductor wafer is compared with a critical discrimination level (threshold level). A point at which these two level coincide is determined to determine the width of a pattern to be measured. However, with a recent tendency toward higher packing density of LSIs and VLSIs, micropatterning and high precision patterning of a semiconductor wafer are becoming daily routines. For this reason, a higher resolution of a device for measuring a width of a semi-conductor pattern is required. More specifically, a resolution of 0.1.mu. or greater is required. However, with the conventional method utilizing an optical microscope, the magnification is limited and limitation of the resolution is about 1/4 the wavelength, both being unsatisfactory. With the conventional method utilizing a laser, high-precision measurement cannot be performed due to the resolution limit of a beam spot diameter of a laser, or variations caused by the difference between the resist pattern and the semiconductor pattern after etching. With the conventional method utilizing an SEM, a reading error is generated during measurement of a dimension of an enlarged image of a pattern using a scale. In the case of the conventional method of aligning cursors displayed on the screen with the edges of the enlarged image as a pattern to be measured, variations in alignment by different operators present a problem. In the conventional method wherein a level of a signal obtained from an SEM is compared with a critical discrimination level, the measurement results are different depending upon the setting of the critical discrimination level. Thus, a point at which the signal from the SEM starts changing or stops changing cannot be determined. That is an upper edge or a lower edge of the pattern can not be detected, so high-precision measurement cannot be performed.